The present invention relates to a length measuring system which permits accurate length measurement for a long magnetizable material such as a steel cable while it is running in an axial direction.
As a method for measuring the length of a long material while it is running, it is known to measure the number of revolutions per unit time of a measuring wheel or an endless belt which rotates or runs by friction with the running material, and determine the length from the number of revolutions. However, such a method has a shortcoming in that some measuring error is inevitable owing to possible slip between the running material and the measuring wheel or the endless belt. Such a friction type method has another problem in that it cannot be applied to any material having an uneven surface such as corrugated pipes and strand cables because the number of revolutions measured does not represent the length of the material accurately.
As a solution to the above-mentioned problems, a length measuring system has been used which includes a mechanical marking means and a mark reading means arranged at a fixed distance along the running direction of the material, the marking is done synchronously with the mark detection so that the length of the material can be measured by multiplying the number of mark detections by the distance between the marking means and the mark reading device. Such a conventional system has an inherent shortcoming in that there is some difficulty in marking the running material accurately and this difficulty inevitably causes some amount of error in the reading of the mark. The measuring accuracy can naturally be increased by increasing the distance between the marks, but this would result in an undue increase in the floor space requirement and could not completely eliminate error.